A related automatic transmission has been proposed in which a one-way clutch and a second brake are arranged in parallel. During take-off, forward first speed is established by applying a first clutch, and automatically applying the one-way clutch according to the rotation speed of a third carrier. When coasting (i.e., when the engine brake is applied), the second brake, which is arranged parallel to the one-way clutch, is applied instead of the one-way clutch (see PTL 1).
[Citation List]
[Patent Literature]
[PTL 1]
Japanese Patent Application Publication No. 2008-121843
In an automatic transmission such as that described in the PTL 1, forward first speed is established by applying the first clutch and the one-way clutch, so the one-way clutch automatically applies when the braked third carrier tries to rotate in one direction.
However, when the torque applied to the one-way clutch suddenly increases, such as during take-off, particularly during a sudden take-off, when the one-way clutch is apt to slip for some reason, such as when the temperature is extremely low so the viscosity of the oil is high and the oil film between the roller and the race of the one-way clutch is difficult to cut through, that torque may overpower the one-way clutch, such that the one-way clutch starts to slip and the third carrier is no longer able to be braked.
In a normal inexpensive one-way clutch, a simple cylindrical roller is inserted between an outer race and an inner race to stop relative rotation between the outer race and the inner race. Thus, One way to prevent the one-way clutch from slipping is to increase the surface pressure and make it easier to cut through the oil film by using a roller having a shape that applies a large amount of frictional force between it and the inner race, such as a reverse crowning roller (an hourglass-shaped roller) that becomes increasingly larger toward both ends.
However, preventing slipping by improving the structure of the one-way clutch in this way makes it difficult to reduce costs because the one-way clutch itself is expensive, as well as makes it difficult to reduce the size because this one-way clutch is large.